Field of the Invention
Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus incorporating the fixing device.
Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of a photoconductor; an optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a development device supplies toner to the electrostatic latent image formed on the photoconductor to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the photoconductor onto a recording medium or is indirectly transferred from the photoconductor onto a recording medium via an intermediate transfer belt; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Such fixing device is requested to shorten a first print time required to output the recording medium bearing the toner image onto the outside of the image forming apparatus after the image forming apparatus receives a print job. Additionally, the fixing device is requested to generate an increased amount of heat before a plurality of recording media is conveyed through the fixing device continuously at an increased speed.
To address these requests, the fixing device may employ a thin endless belt having a decreased thermal capacity and therefore heated quickly by a heater. FIG. 1 illustrates a fixing device 20R1 incorporating an endless belt 100 heated by a heater 300. As shown in FIG. 1, a pressing roller 400 is pressed against a tubular metal thermal conductor 200 disposed inside a loop formed by the endless belt 100 to form a fixing nip N between the pressing roller 400 and the endless belt 100. The heater 300 disposed inside the metal thermal conductor 200 heats the entire endless belt 100 via the metal thermal conductor 200. As the pressing roller 400 rotating clockwise and the endless belt 100 rotating counterclockwise in FIG. 1 convey a recording medium P bearing a toner image T through the fixing nip N in a recording medium conveyance direction A1, the endless belt 100 and the pressing roller 400 apply heat and pressure to the recording medium P, thus fixing the toner image T on the recording medium P.
Since the metal thermal conductor 200 heats the endless belt 100 entirely, the endless belt 100 is heated to a predetermined fixing temperature quickly, thus meeting the above-described requests of shortening the first print time and generating the increased amount of heat for high speed printing. However, in order to shorten the first print time further and save more energy, the fixing device is requested to heat the endless belt more efficiently. To address this request, a configuration to heat the endless belt directly, not via the metal thermal conductor, is proposed as shown in FIG. 2.
FIG. 2 illustrates a fixing device 20R2 in which the heater 300 heats the endless belt 100 directly. Instead of the metal thermal conductor 200 depicted in FIG. 1, a nip formation plate 500, disposed inside the loop formed by the endless belt 100, presses against the pressing roller 400 via the endless belt 100 to form the fixing nip N between the endless belt 100 and the pressing roller 400. Since the nip formation plate 500 does not encircle the heater 300 unlike the metal thermal conductor 200 depicted in FIG. 1, the heater 300 heats the endless belt 100 directly, thus improving heating efficiency for heating the endless belt 100 and thereby shortening the first print time further and saving more energy.
However, the endless belt 100 shown in FIG. 2, as it is not supported by the metal thermal conductor 200 unlike the endless belt 100 shown in FIG. 1, is exerted with various stresses. For example, as shown in FIG. 3A, as the pressing roller 400 rotating in a rotation direction Q1 frictionally slides over the endless belt 100 pressed against the pressing roller 400 by the nip formation plate 500, friction between the pressing roller 400 and the endless belt 100 exerts shear forces indicated by arrows S1 and S2 to the endless belt 100. As shown in FIG. 3B, if the endless belt 100 is skewed in a direction K1 as it rotates, a lateral edge of the endless belt 100 in the axial direction thereof comes into contact with a belt holder 600 that regulates movement of the endless belt 100. Accordingly, as the lateral edge of the endless belt 100 frictionally slides over the belt holder 600, shear forces indicated by arrows S3 and S4 are exerted to the lateral edge of the endless belt 100. As shown in FIG. 3C, if the endless belt 100 is formed into an ellipse in cross-section to facilitate separation of a recording medium from the endless belt 100, the endless belt 100 has different curvatures at positions X and Y and therefore is exerted with a bending force repeatedly.
Those forces generate various stresses that may be concentrated on both lateral ends of the endless belt 100 in the axial direction thereof. As a result, both lateral ends of the endless belt 100 are susceptible to damage or breakage, degrading durability of the endless belt 100.